It is interesting, but is it going to add much to the game as a whole?
Not saying it in of itself is bad, but there are pros and cons to such a thing.
If the damage system is simple, then the weapon concerns are simple, since they'd do about the same damage to everything. This makes the progression of newer and better weapons for each stage of play relatively easy to design and control. However, it simplifies things down to basic attack strategies, and doesn't offer much depth in that aspect of gameplay.
If the damage is detailed, then what weapon (and ammo) you have becomes more of a concern. This adds a layer of complexity that can make things more engaging and advanced, to where the weapon with the highest damage stat isn't automatically better. However, this adds another dimension of game design and balancing, which slows things down as developers try to deal with more factors. This also makes it hard for the player to keep up if the need to upgrade before each stage is too rapid. Not only will players need to carry more weapon types to optimally handle the wide variety of circumstances they'd run into (if they want to be optimal), but also have to figure out and rebuild a new set of weapons as the power of the encounter slowly creeps up.
Right now, we have a lot of weapons, but none of them are particularly designed to work well with one enemy vs another. Some happen to work well vs some types, out of how the game mechanics are; but as far as I know, the game doesn't have something like a Ogre Slaying Knife that does +9 vs Ogre type enemies (if there were Ogres.. or knives.. or whatever the hell a "+9" is).
Also...
And spiky balls against goblins..
What do you mean by this?
Sorry, no support, Spiky balls are already effective against goblins anyway
Some might think the above, but based on the post before, it sounds like you want to have spike balls do less damage to Goblins.
